1. Field of the Disclosure
The disclosure relates to a device and method for positioning a substrate, lying on a print table and to be printed in accordance to a screen printing method, with respect to a print template of a screen printing device.
2. Related Technology
The screen printing method belongs to the classical print-through methods by means of which very exact and also fine print patterns can be attained. Such patterns are necessary, for example, for the conductor arrangement in the case of solar cells. Thus, in practice, these are applied with the screen printing method. In order to attain the desired exactly positioned printing of the conductors on the substrate, a very exact positioning is necessary, which should not exceed a positional deviation of ±15 μm. Beyond this, high productivity should also be attainable; that is, short cycle times with as far as possible automatic operation.
Conventionally, the delivered substrate to be printed is placed on the print table of the screen printing device and fixed by means of a vacuum device through suction. The screen carrying the print template or the print pattern is then placed above the print table and lowered on to the substrate for carrying out the screen printing. Due to various circumstances, in particular through the inexactitude of the transport and placing device, upon fixing of the substrate there arise, however, deviations of the relative disposition between screen and substrate to be printed which exceed the necessary accuracy. It is thus necessary to detect the corresponding deviation and in dependence upon the detected deviation correspondingly to alter the relative disposition of substrate and screen. Expediently, this is achieved in general by means of rotating and/or displacing the screen with respect to the substrate already fixed on the print table. The print table may also be displaced and rotated. Finally, a mixed form can also be employed.
Thus, of significance is a very exact determination of the actual disposition of the substrate on the print table. Since the form of the substrate, as it leaves its production process, is very exactly known, it is sufficient to exactly detect certain predetermined edge sections of the substrate in their disposition with respect to the print table, whereby such an edge section is characterized by a rim; that is, an abrupt transition from substrate to print table.
A screen printing machine of the known kind is disclosed in DE 40 37 678 A1. With this device a material web to be printed is guided to the screen printing device by means of a known web guide and transport device, which screen printing device includes at least one displaceably mounted screen and a device for the reading of alignment marks or the like for multi-colour printing. The displaceably mounted screen is connected with servomotors which are controlled by at least one monitor camera reading the alignment marks or colour displacements, wherein the pulse generator thereof controls the servomotors via a computer or the like.
Disadvantageous with this device is that the displacement of the screen is effected corresponding to the relative disposition of two or more colour print layers lying upon one another, through which a correction of the screen position is possible only after printing has taken place. This is due to the fact that the material to be printed is of a continuous material web, as a result of which no positioning of the screen with regard to a fixed point on the material to be printed itself can be effected.
In accordance with DE 692 30 099 T2 a substrate to be printed is fixed on a print table. Both the substrate and also the screen to be employed possess reference marks. Above the print table an observation device is so mounted that it can view the print table, in particular the reference marks on the substrate mounted on the print table, and can store the position of the reference marks. If the screen or the template is now moved over the substrate to be printed, the observation unite can thus correspondingly determine the position of the reference marks of the screen and from this the relative disposition of the screen with respect to the substrate on the print table. By means of an appropriate correction of the position of the screen, the reference marks of substrate and screen can thus be brought into register.
Disadvantageous here is that for reading the reference marks of the substrate the screen must in each case be removed from the region viewable by means of the observation unit. This signifies a reduction of the speed of the production process.
Conventionally, the substrate and the surrounding region of the print table are well illuminated and the actual disposition of the substrate or the actual disposition of the predetermined edge sections of the substrate is detected by means of a photoelectric detector such as in particular a CCD camera. The electrical signal characterizing the actual disposition of the edge section is processed, and by means of signals generated in the processing the relative adjustment of the screen with respect to the substrate fixed on the print table can be achieved.
With a commercially available positioning system in the solar cell field, by means of a moveable CCD camera, the disposition of the substrate placed on the print table and fixed is achieved outside the actual print region by means of successive travel to the predetermined edge sections. After detection of the various edge sections the disposition is calculated by coordinate calculation. Together with the calculation and the subsequent adjustment of the screen, the print table is moved into the print position. This manner of proceeding is very time consuming and requires numerous moveable parts. Beyond this, considerable masses must be moved. Further, a very complex calibration is necessary, in particular through the carrying out of test prints. If two moveable print tables are employed, on which printing takes place intermittently, different print results can arise.
To overcome certain problems with the known positioning system, in a further development the CCD camera is also used for detecting the disposition of the screen, so that the positioning of the screen with respect to the substrate is somewhat simplified, in particular the initialization or calibration is simplified. However, the cycle times are even greater and the device overall more complex and contains even more moveable parts.
With a further conventional manner of proceeding with two print tables, which are horizontally moveable, there are provided in each case cameras associated with each predetermined edge section for detecting the actual disposition of the substrate. In general, three camera positions are sufficient, but the above-mentioned coordination problems arise to a greater degree, through which different print results arise, whereby also the mentioned disadvantages of the numerous parts to be moved also emerge.